To resolve the shortage of Internet protocol (IP) address, increase network security, and block malicious activities coming from the hosts of external networks, most networking equipment is connected to an external network or another networking equipment through a network address translation (NAT) device. An NAT device may be a device implementing at least NAT. However, data transfer between one networking equipment and another networking equipment may fail if the NAT device cannot be traversed. Therefore, NAT traversal has become a crucial issue to be resolved. According to a method, NAT traversal is achieved through a relay server. Such method has higher transmission success rate but lower transmission efficiency and incurs higher transmission cost. For example, extra bandwidth or longer transmission time is required. According to another method, NAT traversal is achieved through a peer-to-peer (P2P) connection. Such method has better transmission efficiency and requires less bandwidth. However, when the method is used in an NAT device implementing symmetric NAT, transmission success rate is low. Therefore, it has become a prominent task for the industries to provide a method for NAT traversal which produces higher transmission efficiency, incurs lower transmission cost and achieves higher transmission success rate.